You are able to load separate scenes by programming it in. Would be triggered by a proximitytrigger and then load. As for the textures you should be able to define the resolutions within the engine depending on what geo you select. That way the things that are the farthest away can have the least amount. Also the idea of occlusion culling could be utilized to keep the draw calls down and have your fps running much higher.

Silverfeather. You might want to look into using proxies to lighten your solar scene. Within C4D if I have a lot of objects within one scene I have multiple versions of a object that I externally reference with hi and lo settings that I trade out prior to rendering...